This invention relates to a method and apparatus employing computerized tomography for diagnosis and stereotactic surgery. While the invention will be disclosed with particular reference to the requirements of brain surgery, it will be apparent that the invention may advantageously be employed for other procedures.
Stereotactic surgery is a sub-specialty of neurosurgery and defines a class of operations in which probes, such as cannulae, needles, forceps or electrodes are placed into brain regions or anatomical targets that are not visible on the surface of the brain. The general location of these regions is determined by measurements from landmarks visualized by X-ray or other means, such measurements being based on atlases derived from anatomical studies and autopsy. Because of anatomical variability, more precise location in any single patient may be determined by physiological responses in that patient. The degree of success in stereotactic surgery depends upon the experience of the surgeon as well as the precision of the stereotactic instrument and radiologic brain imaging technique.
A stereotactic instrument is a guiding device used in human neurosurgery for the purpose of directing an instrument to a specific point within the brain by radiographic or other visualization of landmarks, through a small opening in the skull. Stereotactic instruments are constructed to afford the surgeon reliably reproducible accuracy in placing instruments into target areas. Proper positioning of the probe is often verified by x-rays to control errors in calculation and to correct deflection of the probe during insertion. Physiologic parameters may be used to further define the optimal target.
At the prresent time, stereotactic instruments are used most frequently, but not exclusively, in the following operations.
Thalamotomy for parkinsonism and other types of tremor, PA0 Electrode implantation for epilepsy, PA0 Needle and/or magnet insertion for aneurysm thrombosis, PA0 Thalamic or subthalamic operations for involuntary movements such as chorea or hemiballismus, PA0 Ablation of deep cerebellar nuclei for spasticity, PA0 Cingulotomy and thalamic or subthalamic surgery for pain, PA0 Mesencephalotomy for pain, PA0 Ablations for subcortical temporal lobe structures for treatment of epilepsy, PA0 Psychosurgical procedures, PA0 Implantation of depth stimulating electrodes for pain, PA0 Insertion of forceps or needle for obtaining biopsy specimens, PA0 Foreign body removal, and PA0 Implantation of radioactive material PA0 Biopsy or treatment of tumors.
The list is presented only to give examples for some applications. It is not required to hit a point in space, but to hit a volume or make a lesion within a mass. The purpose of stereotactic apparatus is to guide the advance of an electrode or other probe accurately and in a controlled fashion to a given point in space, relative to the apparatus, the stereotactic target. Thus, when the apparatus is attached to the skull, the probe can be advanced to a given geographical point within the cranial cavity, near the base of the skull, or in the spinal canal. As generally employed, the ventricles or cavities within the brain or other cerebral landmarks are identified roentgenographically or by other means and, by consulting an atlas or other table, the mean distance and direction between the visualized landmark and a given anatomical target are measured. The probe is then inserted to the stereotactic target, that is, the point in space within the cranial cavity which is calculated from the distance and direction between the visualized landmark and the desired target in relation to the coordinate system of the stereotactic apparatus. It is recognized that there is considerable anatomical variability in brain size and shape so that the target point is identified from the atlas or table is only approximate. Usually, where possible, physiological verification may also be obtained. One must distinguish between the anatomical accuracy, which is inexact because of the variability of brains, and the mechanical accuracy, which is a function of the precision of the stereotactic instrument.
In the utilization of computed tomography for stereotactic surgery some targets may be directly visualized in an image, such as a brain tumor.
As previously stated x-ray images of the brain are currently used in neurosurgery to locate the pertinent landmarks. In principle a series of images in orthogonal planes allows the neurosurgeon to determine landmark coordinates. Unfortunately a landmark may not be readily identifiable because of the poor density resolution of conventional x-ray images and uncertainties about the head orientation.
Computerized tomography provides a new imaging technique which not only has high density resolution capabilities, but also provides a quantitative information about the anatomy. In accordance with the invention, computerized tomography can be integrated in a neurosurgical procedure to provide major improvement in target identification.
Basic concepts of CT scanning and the displays related thereto are described in U.S. Pat. No. 3,778,614, issued Dec. 1, 1973, the disclosure of which is specifically incorporated herein by reference.
A comprehensive analysis of the integration of computerized tomography CT in neurosurgery requires a definition of the differences between surgical requirements and the scanning configuration and data presentation in commercial CT scanners which are designed to satisfy diagnostic requirements.
The basic information obtained from a conventional CT image is the value of local tissue density which is used for diagnosis of tissue anomalies. The spatial density distribution generates the information about the anatomy and the location and dimensions of tissue anomalies. Thus for diagnostic purposes, spatial resolution in the image plane, as well as thickness of the tissue "slice" covered in each scanning, are selected to achieve a maximum sensitivity in tissue density discrimination. This contrasts with the requirements of a surgical procedure, where the anatomy and in particular the outline of body organs is the dominant parameter to determine either target point or landmark location. Scanning parameters and image reconstruction algorithms must then be selected to obtain a maximum precision in target location measurement while tissue density discrimination may become of secondary importance.
In a normal CT scanner procedure for diagnostic purposes a multiplicity of scans may be taken to explore the entire region of the brain as well as to determine the three-dimensional properties of the tissue element under scrutiny. The distance between scanning planes or slices and the thickness and number of slices depend upon the specific information which is sought by the clinician in each particular case. In a surgical procedure the sequence of scans must provide the spatial coordinates of a target point. Thus, in a general case, the element of volume of interest must be explored uniformly with a sequence of scans at intervals selected to maintain a uniform spatial resolution throughout the element of volume.
With respect to the dimensions of the volume to be scanned, for diagnostic purposes a series of total scans of the head are necessary, while in a surgical procedure the scans may be limited to the region of interest, because by the time the patient is brought into the surgical room, the diagnosis has been completed and conventional scan results are available to the surgeon. Dimensions of the order of 5 cm. of the volume to be imaged during the surgical procedure are adequate for the brain. The surgical scanner can then be designed for partial scanning with two important advantages. First, the limited extent of the partial scanning region makes it possible to achieve a high spatial resolution without increasing the total x-ray dose. Second, size and weight of the gantry of a scanner designed for partial scanning in such a small region may be drastically reduced compared to a conventional scanner.
The above considerations refer primarily to the imaging logic and scanning modality. Additional important considerations have to be made regarding surgical instrumentation and procedure as well as patient handling. First of all, the stereotactic guide and the head support must be designed to minimize their interference with the x-ray beam throughout the scanning sequence. The design of the stereotactic guide can easily be arranged to keep the controls and supports outside of the scanning planes. On the other hand conventional head holders are not so easily adapted to this system because of the relatively small degree of freedom in the location of constraining pins which hold the skull in the proper position. If the pin structure must cross the scanning plane, considerable care has to be taken in the selection of materials and in the design of the support to avoid the creation of strong artifacts throughout the image. However, the design of these surgical components is only a part of the total problem of satisfying both surgical and scanning requirements. It is well known that the image reconstruction requires the acquisition of data over a rotation of the x-ray source of at least 180.degree. in the scanning plane. This has resulted in a closed configuration of all commercial scanners with an opening whose dimensions are dictated by the cross section of the human body. The closed configuration and the position of the scanning plane relative to the patient support makes a commercial scanner hardly suitable for stereotactic surgical procedures since it interferes with the surgeon's access to the surgical area. Both size and shape of the scanner gantry are thus an important factor in the design of an integrated surgical system.
In addition, patient handling procedures for diagnostic purposes may not be suitable for surgical applications. In a commercial scanner, with the exception of gantry tilting, it is the patient support that undergoes axial as well as vertical motion to position a given section of the patient body in the scanning plane. In a surgical procedure a preliminary phase involves arrangement of the patient in a position which satisfies both the surgical and scanning requirement. This phase may involve the control of position and orientation of both scanner gantry and patient support. However, once the preliminary phase is over and the patient's head is locked in its support, the ideal situation is to keep the patient immobile and to confine all motions to the instrumentation including the indexing of scanning positions during the scanning sequence.
In accordance with the invention, the image reconstruction algorithm and the orientation of the image planes are selected to optimize primarily the presentation of tissue anatomy rather than tissue characteristics. In addition, the scanning procedure is limited to a partial scanning of the volume of interest with a spatial resolution uniform in the scanning plane as well as perpendicular to the scanning plane. A low scanning speed to optimize image quality must be selected as a trade-off between x-ray dose within the region of partial scanning and total scanning time of the volume of interest. The dimensions of the volume explored in the partial scanning procedure is selected as a trade-off between surgical requirements and amount of data and computational time. Head holder and stereotactic guide are preferably designed to minimize their interference with the scanning procedure throughout the volume of interest. The gantry is designed to minimize obstructions to the surgeon's access to the surgical area and provide maxiam flexibility in patient positioning. Translations and angular orientations required by the scanning procedure are implemented in the scanner gantry rather than in the patient support. In a preferred embodiment of the invention, safety features are built into the scanner for possible emergencies, including the rapid removal of the gantry from the patient support should the need arise.
Additional features are preferably included to monitor the actual surgical procedure. Upon completion of the target identification phase and adjustment of the orientation controls of the stereotactic guide, the probe is driven into the brain region to reach the depth of the target point. The penetration has to be monitored by measuring the coordinates of the probe tip position prior to reaching the target point. Thus the x-ray system of the scanner is used to monitor the probe tip position at prescribed points of the probe trajectory.
In conjunction with certain of the above-noted objectives, the present invention includes a preferred gantry structure having a generally open configuration which provides better access to the patient and reduced interference by the gantry and associated components with the scanning beam. The approach herein is a basically open C-shape support in contrast to the traditional closed circular support. Two arms of the C-shape define an arc nominally 180.degree. but actually somewhat greater for reasons of practicality.
The basic scanning procedure is to scan across one plane at a time through the specimen by directing the X-ray beam through a succession of parallel or angled orientations all within the particular scan plane, and then to move the scan plane sequentially at 1.5 mm or other selected increments along an axial coordinate perpendicular to these planes. Traditionally the scan is achieved by moving the source in a straight line above the specimen with a similar parallel movement of a detector below the specimen, or by moving the source circumferentially around the specimen with an identical movement of the detector, such that the source and detector remain diametrically opposed with a constant distance between them as the scan occurs. In circular scan procedures the traditional prior art support structure is a closed circular frame around which the source and detector move, thus requiring the patient to be moved axially into the circular frame. The source is caused to move along a circular path with the radiation beam directed radially inward across the center of the circle to a detector at the opposite side of the circumference, and the detector is simultaneously caused to move in the opposite direction. Various arrangements have been used so that after each plane is scanned, the source and detector are circumferentially or laterally returned to their starting position and are axially moved to a next adjacent plane to be scanned.
In the present invention detection is achieved with a new structural arrangement which provides better continuity of detection by scanning during forward movement of the detectors and while the detectors move backwards to return to their starting position, and by using a pair of detectors operated one behind the other in the direction of motion.
The lead detector moves to the rear of the other after each of many incremental movements along the circumferential path, while the source moves essentially continuously through the entire circumferential cycle. The pair of detector units still constitutes a single detector means remaining angularly displaced 180.degree. from the source, but also angularly moveable the same manner as the source. Later the process is repeated in the opposite direction so that there is no need to recycle or reposition the detector when the source begins its reverse circular motion about the frame. The preferred arrangement devised is to have a first detector near the end of its operative cycle followed closely by a second detector which moves into the position and role of the first so that the first can be repositioned behind the second when the second nears the end of its operative cycle.